The present invention relates to a method of mass and/or ion mobility spectrometry and a mass and/or ion mobility spectrometer.
Ion mobility separators are known in which ions are separated according to their ion mobilities. In order to improve the duty cycle of ion mobility separation, the ions may be accumulated in a trapping region upstream of the ion mobility spectrometer or separator device. As all the ions are stored in this trapping region prior to release into the drift tube, the space charge capacity of this upstream trapping region can ultimately limit the performance of the ion mobility spectrometer or separator device. Excessive amounts of charge in the trapping region can lead to detrimental effects on performance of the downstream analyser. For example, if the charge capacity of the device is exceeded then ions may be lost from the trapping region. There may be undesirable mass and/or charge discrimination in the portion of the population of ions lost from the trapping region due to the nature of the confining force provided by the RF confinement field. In addition, there may be fragmentation of molecular ions as ions are pushed closer to the confining electrodes due to mutual space charge repulsion.
One way to increase the charge capacity is to extend the axial length of the trapping region. However, the resolution of the downstream ion mobility spectrometer or separator will be reduced if an axially extended ion packet is introduced into the ion mobility spectrometer or separator. Best performance is preferably achieved when the packet of ions introduced into the device has the minimum practical axial distribution prior to ion mobility separation.
Accordingly, it is desired to provide an improved mass spectrometer and method of mass spectrometry.